In the recent drive for higher integration and operating speeds in LSI devices, it is desired to miniaturize the pattern rule. Great efforts have been devoted for the development of the micropatterning technology using deep-ultraviolet (deep-UV) or vacuum-ultraviolet (VUV) lithography. The photolithography using KrF excimer laser (wavelength 248 nm) as the light source has already established the main role in the commercial manufacture of semiconductor devices. The photolithography using ArF excimer laser (wavelength 193 nm) is at the start of its application to the advanced micropatterning process on a commercial scale. For the ArF excimer laser lithography, however, partly because the succeeding technology has not been established, it is strongly desired to improve the performance of resist material in order to gain a further stretch of resolution. Efforts are also made to develop the immersion lithography process that intends to gain a stretch of resolution by interposing a high refractive index liquid between a resist coating film and a projection lens. There is a need for a resist material meeting such requirements.
The requisite properties for the resist materials complying with the ArF excimer laser lithography include transparency at wavelength 193 nm and dry etch resistance. Resist materials comprising as a base resin poly(meth)acrylic acid derivatives having bulky acid-labile protective groups as typified by 2-ethyl-2-adamantyl and 2-methyl-2-adamantyl groups were proposed as having both the properties (JP-A H09-73173 and JP-A H09-90637). Since then, a variety of materials have been proposed. Most of them commonly use resins having a highly transparent backbone and a carboxylic acid moiety protected with a bulky tertiary alkyl group.
While prior art resist materials adapted to the ArF excimer laser lithography suffer from many problems, the most serious problem is a decline of resolution due to excessive diffusion of the acid generated by the photoacid generator. Generally speaking, in the ArF excimer laser lithography, the acid generated upon exposure triggers deprotection reaction on the base resin which proceeds during heat treatment following exposure (post-exposure bake or PEB). Migration of the acid occurs during PEB. Since chemically amplified resist materials depend on the function of the acid to act as a catalyst to promote deprotection reaction, moderate acid migration is necessary. However, acid migration degrades an optical image, indicating that excess acid migration detracts from resolution. To comply with the outstanding demands for a further size reduction in the ArF excimer laser lithography and a higher resolution due to a good command of the immersion lithography, there is a need for a resist material featuring controlled acid migration and higher resolution capability.
In the ArF excimer laser lithography, on the other hand, it has been practiced to reduce the thickness of resist film for the purpose of achieving a higher resolution. Since the resist pattern tends to collapse down as the feature size is reduced, thinning of resist film is a logical approach. Thinning of resist film means that the film itself is improved in transmittance, which is advantageous for resolution enhancement. This eliminates a need for improvement in transmittance of resist material. However, since thinning of resist film undesirably deprives the film of etch resistance, it is necessary to improve the etch resistance of resist material.
For high-energy radiation with very short wavelength such as EB and x-rays, since light elements such as hydrocarbons used in resist materials have little absorption, resist materials based on polyhydroxystyrene have been under investigation.
In practice, resist materials for EB have been used in mask-writing application while the mask production technology is lately regarded problematic. Since the g-line age, reducing projection aligners have been used, with their demagnifying power being ⅕. Recently, a demagnifying power of ¼ is employed in accordance with the enlargement of chip size and the increasing aperture of projection lens. Not only a reduction in line width as a result of progress of micropatterning, but also a reduction in line width as a result of magnifying power change pose serious problems to the mask production technology.
The exposure system for use in mask production has changed from a laser beam exposure system to an electron beam (EB) exposure system in order to increase the line width accuracy. Since definition of a finer feature size becomes possible by increasing the accelerating voltage in an electron gun in the system, the accelerating voltage has increased from 10 keV to 30 keV. An accelerating voltage of 50 keV now becomes the mainstream.
In conjunction with the increasing accelerating voltage, a lowering of resist film sensitivity becomes a problem. As the accelerating voltage increases, the influence of forward scattering within the resist film is reduced so that the contrast of electron writing energy is improved, resulting in improvements in resolution and dimensional control. However, since such electrons can penetrate straight through the resist film, the sensitivity of the resist film lowers. Since the mask exposure system carries out exposure by direct writing along a continuous stroke, the lowering of resist film sensitivity undesirably leads to a lowering of productivity.
To meet the recent demand for finer feature size, research efforts have been made to further increase the accelerating voltage. EB exposure at an accelerating voltage of 100 keV is under consideration. There is thus a need to compensate for a concomitant decline of sensitivity.
While the pattern rule to be applied to devices is being reduced, the KrF excimer laser lithography survives nearly to a device size of 100 nm, and the ArF lithography starts application from 90 nm, enables processing nearly to a size of 60 nm, and is expected to reach even a size of 40 nm in the case of immersion ArF lithography, namely when combined with a high refractive index liquid having a refractive index of at least 1.0 interposed between the resist film and the projection lens. Research is also made on processing of a finer line width by exposure to soft x-ray (or EUV) having a wavelength of 5 to 20 nm, which is expected to process a pattern with a line width of nearly 20 nm. With a progress toward finer size patterns on wafers or processable substrates, the demand for a higher resolution capability of resist material becomes stronger. The same applies to the EB image writing for mask production, and there is a strong demand for a higher resolution capability. Because of ¼-demagnifying power projection onto a wafer, the line width for mask processing is 4 times the line width on the wafer. A progress in finer size processing requires higher resolution performance factors including not only the line width, but also the line width uniformity. This necessitates a resolution enhancement on the resist material side.
In the case of mask processing, a resist film is thinned for the purpose of enhancing the resolution capability thereof as in the case of device processing. Since the substrate used in mask production is flat and the thickness of a processable substrate (e.g., Cr, MoSi or SiO2) is predetermined so as to achieve a desired percent light shielding and phase difference control, it is impossible to improve the processable substrate. Also in the case of mask processing, the resist material must be improved in dry etching resistance before the resist film can be thinned.